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In order to improve the automatic generation control (AGC) performance of thermal generators, this paper presents a stochastic model predictive control (SMPC) approach for a battery/flywheel hybrid energy storage system (HESS) to distribute power. The approach combines an adaptive Markov chain for power demand prediction of HESS, a scenario tree generation and model predictive control strategy. To develop an effective prediction model to deal with the randomness of a thermal generator in response to AGC command, a Markov chain is used to describe the randomness of the HESS power demand, and a posteriori information is used to adapt to the fluctuation of the AGC command. A scenario tree generation approach is proposed to make better use of the Markov probability matrix. Based on these efforts, an SMPC approach is proposed for HESS energy management. Simulation results show that the regulation performance of the proposed approach outperforms conventional approaches, and that its performance is close to the model predictive control strategy with prescient information of the future power demand. In addition, compared with other approaches such as rule-based strategies, it does less damage to the battery and yields higher annual average net benefits in the whole life cycle.

Hybrid circuit breaker (HCB) has shown good performance for interrupting dc fault current. However, a massive number of IGBTs in series is needed. These will lead to high costs and significant difficulty in balancing IGBT dynamic voltage. This paper proposes a bidirectional hybrid circuit breaker based on a cascaded half-bridge submodule structure to reduce the overall costs and implementation difficulty of HCB. The number of IGBTs is reduced by 50%, and the operating speed is fast enough. A model of a four-terminal modular multilevel converter (MMC) system with a proposed hybrid circuit breaker in different stages is developed to describe the fault blocking process more clearly. The performance of the proposed HCB is validated and compared with other HCB structure through both theoretical derivation and time-domain simulation.

A new perspective of impedance matching is proposed to study the power flow operation of the three-phase four-wire (3P4W) unified power quality conditioner (UPQC) system. Specifically, the UPQC system is equivalent to an adjustable impedance network, when external conditions change, such as grid voltage sag/swell and unbalanced loads, the node impedances in the network will be dynamically matched. On the one hand, the three nodes of the series converter (SC) will be matched as positive or negative impedances with the change of grid voltages, which means that the SC operates in a rectifier or inverter state to absorb or transmit the energy. On the other hand, to compensate the unbalanced load currents, the three nodes of the parallel converter (PC) will be matched as positive and negative impedances at the same time, which means that the PC operates in both rectifier and inverter state to absorb and transmit the energy. After matching the node impedances, the system's output node impedances are equal to the load impedances, thus achieving the power balance of the whole system. Experimental results of the power flow and impedance matching from a laboratory-scale prototype hardware are presented to evaluate the correctness of the impedance analysis.

Energy storage systems (ESSs) are promising solutions for the mitigation of power fluctuations and the management of load demands in distribution networks (DNs). However, the uncertainty of load demands and wind generations (WGs) may have a significant impact on the capacity allocation of ESSs. To solve the problem, a novel optimal ESS capacity allocation scheme for ESSs is proposed to reduce the influence of uncertainty of both WG and load demands. First, an optimal capacity allocation model is established to minimize the ESS investment costs and the network power loss under constraints of DN and ESS operating points and power balance. Then, the proposed method reduces the uncertainty of load through a comprehensive demand response system based on time-of-use (TOU) and incentives. To predict the output of WGs, we combined particle swarm optimization (PSO) and backpropagation neural network to create a prediction model of the wind power. An improved simulated annealing PSO algorithm (ISAPSO) is used to solve the optimization problem. Numerical studies are carried out in a modified IEEE 33-node distribution system. Simulation results demonstrate that the proposed model can provide the optimal capacity allocation and investment cost of ESSs with minimal power losses.

The conventional half-bridge submodule (HBSM)-based multilevel modular converter (MMC) cannot block DC faults. To solve this problem, a novel flexible overhead-line high-voltage direct current (HVDC) transmission converter station topology is proposed in this study, which provides DC fault blocking capability. By adding blocking submodules (SMs) onto the positive and negative DC buses of a conventional HBSM converter station, a rapid fault current blocking can be achieved in the case of a DC bus short circuit. The DC fault blocking principle of the proposed topology and the rapid fault blocking capability of the blocking SM are analyzed. The quantity of required major components is also calculated. In addition, the IGBT, the system loss and the control complexity quantities are comprehensively compared with those of existing topologies. The validity of the proposed topology is demonstrated based on simulation and experimental studies.

Except for pumped storage, other existing electric energy storage technologies are difficult to achieve large-capacity energy storage and not easy to simultaneously meet the requirements in terms of site selection, cost, efficiency, and response. For this end, this paper combines the advantages of maglev technology and vacuum technology, proposes a new type of mechanical large-capacity energy storage technology which is vacuum pipeline maglev energy storage. Based on the idea of vacuum pipeline maglev energy storage technology, we have constructed a case system of vacuum pipeline magnetic maglev energy storage technology. The key technical parameters of the energy storage system, such as the maglev train's weight ratio and speed per hour, the mode of levitation and guidance, the car-track structure, the type and size of the vacuum pipeline, the type of the motor, the process of charging with acceleration, the process of no-load with uniform-speed and the process of generating electricity with deceleration were studied. On this basis, the energy storage capacity and cost of the vacuum pipeline maglev energy storage system were obtained through analysis. By comparing the energy storage capacity and cost of Fengning Pumped Storage Power Station in China, the advantages of vacuum pipeline maglev energy storage technology in economy and technology were verified. Finally, based on the technical characteristics of the vacuum pipeline maglev energy storage system, we analyzed its broad applications in renewable energy power consumption, optimization and upgrade of distribution network structure, urban emergency power supply and pulse power supply.

With the rapid development of distributed power and new energy vehicles, the traditional distribution network is facing enormous challenges. The development of multi-station integration technology is an effective way to solve the above problems. In this paper, a new multi-station integration topology is proposed. The new multi-station integration topology uses soft open point (SOP) as the key equipment of the system, and adopts the hierarchical power supply mode for data center, electric vehicle charging pile and other function stations. Then, based on the new topology of multi-station integration and smooth switching of converter control mode, a coordinated control strategy is proposed. Through the coordinated control strategy to control the working mode of each function station, and then further optimize the system operation conditions. Based on Matlab/Simulink, the simulation model of multi-station integration is established, and two different application scenarios are analyzed. The simulation results verify that the new topology of multi-station integration and its coordinated control strategy effectively improve the voltage level of the distribution network, improves the reliability of sensitive load power supply, and ensure the safe and reliable operation of multi-station integration system.

The current large-scale access to distributed power and the rapid growth of electric vehicles are seriously affecting the reliability of distribution networks. The new multi-station fusion technology based on the flexible interconnection of the distribution network can effectively solve this problem. In this study, we design a multi-purpose station and a multi-function device using a soft normally open point (SOP). A new multi-station integration topology and a coordinated control strategy are developed using an active power signal (APS) and an energy management system (EMS). The coordinated control strategy involves sending out the corresponding operation mode command after the EMS receives the superior instruction or the APS of the local system and coordinating the operation mode of the SOP, energy storage DC/DC converter, and photovoltaic DC/DC converter, thereby ensuring stable and efficient operation of the multi-station fusion system. Finally, two typical cases are simulated to verify the feasibility and effectiveness of the proposed topology and control strategy.

The analysis and calculation of the short-circuit fault current in the DC grid is of great significance to the design and configuration of the converter station and DC circuit breaker parameters. The existing flexible DC system not only includes the modular multilevel converter (MMC) converter, but also needs power equipment such as the fault current limiter and DC circuit breaker. Therefore, the system modeling and short circuit calculation of the multi-terminal DC system after adding the DC circuit breaker are also of great significance for the design of DC power system parameters and the grid troubleshooting ability. In this paper, firstly, the parameters of the four-terminal DC system of the modular multilevel converter (MMC) are simplified, and the analytical solution of the short circuit fault current of the multi-terminal DC system is given. Then, the external characteristics of the cascaded hybrid DC circuit breaker are modeled. Based on the equivalent circuit of the fault current in different stages, the short circuit calculation method of four-terminal MMC system with DC circuit breaker is obtained. This method can effectively describe the overall trend of fault current and provide the basis for the configuration of DC line protection settings and DC circuit breaker related parameters.

In a 100% clean energy town, to meet the energy balance and reduce the impact of power fluctuations on the main grid, in this paper, a hierarchical optimal energy management strategy (EMS) for a hybrid energy storage system (HESS) is proposed. The EMS consists of three layers. To meet the requirement of 100% clean energy, in the upper layer, a HESS economic operation model based on the mixed integer non-linear programming (MINLP) is presented. Then, considering the uncertainty of renewable energy and load, a power prediction model is presented in the middle layer. In addition, to reduce the power disturbance on the main grid caused by the stochastic power, a stochastic model predictive control (SMPC) strategy is implemented to optimize the power allocation of a HESS. In the lower layer, to reduce the power fluctuations of the lithium-ion battery (LiB) when mitigating minute-scale power fluctuations, a HESS optimal power allocation strategy based on Pontryagin's minimum principle (PMP) is proposed. In every control period, each layer is optimized based on the results of the previous layer. Finally, a simulation study is provided to validate the effectiveness of the proposed EMS. The results show that the proposed strategy has good performance in typical scenarios.

A large number of distributed photovoltaic power generations that are connected to the distribution network will have a serious impact on the power quality of the distribution network and load power security. Integrating energy storage and photovoltaic cells into the micro-grid can effectively improve the power quality of distribution network and the reliability of load consumption. This paper is a research on the seamless switching control strategy for micro-grid integrating photovoltaic and energy storage (MIPES). Firstly, it analyzes the boundary conditions of MIPES seamless switching control of grid-connected/islanded modes and studies the output characteristics of photovoltaic cells over a day. The DC bus voltage can be stabilized within a certain range to meet the requirements for MIPES seamless switching of grid-connected/islanded mode. Secondly, this paper analyzes the working characteristics of silicon controlled rectifier (SCR). The results show that when SCR is turned off naturally, the load voltage will rise sharply, affecting the safety of local load. Therefore, a control strategy for SCR switch forced shutdown is proposed to solve the problem of sharp rise of load voltage during grid-connected/islanded conversion of MIPES and to ensure the load security. Finally, relevant simulation studies are carried out to verify the rationality and effectiveness of the proposed control technology.

The widely used flywheel energy storage (FES) system has such advantages as high power density, no environment pollution, a long service life, a wide operating temperature range, and unlimited charging&ndash, discharging times. The flywheel array energy storage system (FAESS), which includes the multiple standardized flywheel energy storage unit (FESU), is an effective solution for obtaining large capacity and high-power energy storage. In this paper, the strategy for coordinating and controlling the charging&ndash, discharging of the FAESS is studied in depth. Firstly, a deep analysis is conducted on the loss generated during the charging&ndash, discharging process of the FESU. The results indicate that the loss is related to the charging&ndash, discharging of power. To solve the problems of over-charging, over-discharging, and overcurrent caused by traditional charging&ndash, discharging control strategies, this paper proposes a charging&ndash, discharging coordination control strategy based on the equal incremental principle (EIP). This strategy aims to minimize the total loss and establish a mathematical model of optimal coordination control with the constraints of total charging&ndash, discharging power, rated power limit, over-charging, over-discharging, and overcurrent. Based on the EIP, the optimal distribution scheme of power charging&ndash, discharging is determined. Secondly, this paper gives the specific implementation scheme of the optimal coordinated control strategy. Lastly, the charging&ndash, discharging coordinated control strategy is verified by examples. The results show that the coordinated control strategy can effectively reduce the loss during the charging&ndash, discharging process and can prevent over-charging, over-discharging, and overcurrent of the system. Overall, it has a better control effect than the existing charging&ndash, discharging control strategies.

CHB is a suitable type of topology for synchronous rectifier application which has two or more independent DC loads. Focusing on the individual DC-link capacitor voltage balancing problems caused by asymmetric parameters of independent DC loads, this paper developed a new dual modulation signals based phase disposition pulse width modulation (PD-PWM) strategy using dynamic carrier bias allocation method. Its main idea is to allocate the bias of the carrier wave dynamically according to the balance situation of the system. Compared with the traditional modulation strategies, this method is more easily to be realized and has much stronger dynamic regulation ability. This modulation strategy is widely applicable to various types of CHB structures. A detailed analysis of the DC-link capacitor charge-discharge profile is performed, and an operation condition based dynamic bias allocation method is described using the cascaded H-bridge rectifier (CHBR) structure of five sub-modules as an example. A simulation model and an experiment platform are developed, and the feasibility and effectiveness of the modulation strategy are verified by simulation and experiment results.

Each arm of modular multilevel converter (MMC) consists of a large number of sub-module (SM) units. However, it also increases the probability of SM failure during the long-term system operation. Focusing on the fault-tolerant operation issue for the MMC under SM faults, the traditional zero-sequence voltage injection fault-tolerant control algorithm is analyzed detailed and its disadvantages are concluded. Based on this, a novel fault-tolerant control strategy based on phase disposition pulse-width modulation (PD-PWM) is proposed in this paper, which has three main benefits: (i) it has carrier and modulation wave dual correction mechanism, which control ability is more higher and flexible；(ii) it only needs to inject zero-sequence voltage in half a cycle of the modulation wave, which simplifies the complexity of traditional zero-sequence voltage injection control algorithms and much easier for implement; (iii) furthermore, the zero-sequence voltage can even be avoided injecting under the symmetrical fault conditions. Finally, the effectiveness of the proposed control strategy is verified with the simulation and experiment studies under different fault conditions.

Lithium-ion capacitor is a hybrid electrochemical energy storage device which combines the merits of lithium-ion battery and electric double-layer capacitor. It is of great importance to monitor the real capacity to evaluate failures of lithium-ion capacitors. Remaining Useful Life (RUL), which is referred to remaining cycle number before reaching its End of Life (EOL) threshold, is a key part in the prognostics and health management and an important indicator of the depletion capacity of lithium-ion capacitor. In this paper, we propose a hybrid neural network which combine with the convolutional neural network and Bidirectional Long Short-Term Memory Network (Bi-LSTM), the data will be used to train this model. Finally, the verifications among different prediction horizons and other methods are discussed. According to the experimental and analysis results, the proposed approach has high reliability and prediction accuracy, which can be applied to battery monitoring and prognostics, as well as generalized to other prognostic applications.

DC line fault is one of the important factors restricting the development of flexible DC system. DC circuit breaker is an effective means to realize the short circuit fault isolation on DC side of flexible DC system. Therefore, it is necessary to add DCCB into the short-circuit current calculation and modeling of MMC-HVDC power grid for comprehensive analysis. Firstly, the external characteristics of the hybrid HVDC circuit breaker and the modular multilevel converter (MMC) are modelled. Then, the modeling of short-circuit fault current in different stages was analysed. The analysis method of short circuit fault current of four-terminal MMC system with DC circuit breaker is obtained. Finally, the simulation results and the calculation results are compared and results show the effectiveness of the short-circuit calculation method proposed by this paper.

In recent years, inspired by the idea from FACTS in ac system, scholars put forward the concept of DC FACTS, and the research and application of DC FACTS equipment for DC distributed network is discussed, such as the DC voltage compensator, DC power flow controller, DC fault current limiter and so on. In order to further improve the cost performance of DC FACTS device, drawing on the research results of AC FACTS multiplex, this paper aims at exploring a new structure for series-parallel multi-function composite controller in dc grid which contains multi-function in power flow control, voltage compensation and fault current limiting. The working principle of series-parallel multi-function composite controller is proposed and the main circuit parameter design method is described. The simulation model is built to show the feasibility of the proposed topology.

This paper proposes a new coordinated operation strategy to improve the low utilization rate of series inverter in traditional unified power quality conditioner (UPQC), and to fully optimize the total capacity of UPQC. When source voltage is in the normal range, the series inverter compensates part of reactive power by reducing the source power factor angle, so as to reduce the capacity demand of shunt inverter. In the case of deep voltage sags, the series inverter always decreases its active power exchange with the shunt inverter by injecting appropriate voltage, and thus lessens energy coupling between the two inverters. In addition, it also presents a capacity design scheme based on the new coordinated operation strategy, which has been proven effective in reducing the total capacity demand up to 27% of compensating device in the most range of load power factor. Finally, the new coordinated operation strategy is validated through an experimental study.

This paper shows the application of the improved droop control strategy in Dynamic Voltage Restorer (DVR) parallel system. Firstly, the traditional droop control strategy for DVR parallel system is shown. And then analyze the different impact on the parallel system by introducing the virtual impedance control, which could reduce the influence of the line impedance. However, the virtual impedance could not completely eliminate the differences of the equivalent output impedance between the parallel inverter units. So, this paper proposes an improved droop control strategy to eliminate the coupling of the active power and reactive power. Based on this control strategy, the circulating current between the parallel units is also suppressed. Simulation and experimental results are presented to verify the validity of this approach.

Lithium-ion capacitor is a hybrid electrochemical energy storage device which combines the merits of lithium-ion battery and electric double-layer capacitor. The precise state of charge estimation is essential to utilize lithium-ion capacitor efficiently. In this paper, the electrical characteristics of lithium-ion capacitor have been studied and the corresponding equivalent circuit model is established. Then, an adaptive square root cubature Kalman filter with variable forgetting factor recursive least square has been proposed to achieve state of charge estimation at each sampling step. The proposed method identifies the model parameters and noise statistics in real time, leading to obtain accurate SOC estimation even if the method is initialized with inaccurate parameters. Estimations under four working conditions are quantitatively studied, the experimental results show that the proposed method has better performance and the corresponding maximum root mean square error is controlled within 1%. Furthermore, the verification of robustness illustrates that the proposed method remains stability and acceptable accuracy when external interference and inaccurate initial SOC are included.

As a new type of energy storage devices with both high energy density and power density, lithium-ion capacitors have exhibited applicable prospects in the field of renewable energy and transportation. In order to explore possible applications of lithium-ion capacitors, equivalent circuit models have been extensively studied. The models are capable of representing the terminal characteristics of a cell. This paper establishes and compares two equivalent circuit models for lithium-ion capacitors: a classical model and a two-branch model. The experimental and simulation results of the two equivalent circuit models are compared under various working conditions to validate the accuracy of the models. A new parameter identification method combining numerical fitting and circuit analytical is developed for the two-branch model. Moreover, the assumptions and preconditions of the two-branch model are deeply analyzed to obtain a general parameter identification method.

The micro-source in micro-grid may be very close distance to the load, and the power output is directly affected by the non-linear load. The harmonic problem caused by the non-linear load of energy-storage micro-source is studied in this paper. The control system is designed using the Modified Sliding Mode Variable Structure Control (MSMVSC) with power electronic technology. The result showed that the voltage loop using the modified sliding mode algorithm can effectively improve harmonic problems.

Supercapacitor is an emerging technology in the field of energy storage systems that can offer higher power density than batteries and higher energy density over traditional capacitors. Supercapacitor will become an attractive power solution to an increasing number of applications, such as renewable energy power generation, transportation, power system and many others, because of its advantages which include high charge/discharge current capability, very high efficiency, wide temperature range, etc. In this paper, the advantages and disadvantages of supercapacitor are discussed and some critical technologies for designing supercapacitor energy storage system are presented in detail. Finally, the role of the supercapacitor in renewable energy power system is discussed and a supercapacitor based uninterrupted power system (UPS) for the wind turbine pitch systems is presented as a design example.